Abstract
1. Experiments were performed to determine whether intercalated disks represent a high or a low resistance to the diffusion of K ions.2. Bundles of sheep ventricular fibres, 0.6-0.8 mm thick and 7-11 mm long, were pulled through a hole in a partition. One half was charged by radiopotassium (K(*)); the other half was washed by inactive Tyrode solution.3. A steady state with respect to the distribution of K(*) along the bundle was reached 6 hr after the exposure to radioactivity.4. On the ;washed' side [K(*)] fell along a relatively smooth curve and did not tend to the value zero towards the cut end of the bundle. The average space constant for the decrease of [K(*)] was 1.55 mm, corresponding to a distance of about 12 x the length of a cell. This indicates that the disk resistance is low and that the fibres are healed over at their cut ends.5. Rhythmical contraction had no detectable effect on the space constant. This may mean either that shortening and thickening of a cell does not effectively mix its content, or that stirring goes on even in resting cells.6. On the assumption of mixing between disks the average resistance of 1 cm(2) of disk to the movement of K ions is 3 Omega. If there is no mixing this value would be even lower.7. The assumption that K(*) distribution within the ;washed' half of the bundle is a consequence of cell-to-cell rather than of extracellular movement is supported by the results of several control experiments.8. It is concluded that the propagation of the cardiac action potential is possible by local circuit currents.